In recent years, from the standpoint of global environmental conservation, in order to regulate CO2 emission, there is a demand for improvement of automobile mileage (weight reduction of automotive bodies). In addition, in order to protect occupants in a crash, there is also a demand for improvement in the crash safety performance of automotive bodies. In order to satisfy both the weight reduction of automotive bodies and improvement in the crash safety performance of automotive bodies, it is said to be effective to increase the strength of steel sheets without decreasing body stiffness and decrease the thickness of sheets to reduce the weight. Thus, high-strength steel sheets are actively used in automotive parts. The effect of weight reduction increases with increasing strength of a steel sheet. Thus, in the automobile industry, for example, steel sheets having TS of 440 MPa or more tend to be used as panel materials for inner sheets and outer sheets.
Many automotive parts made of steel sheets are formed by press working. Thus, automotive steel sheets must have excellent press formability. However, formability, particularly deep drawability and stretch flangeability, of high-strength steel sheets is significantly inferior compared with mild steel sheets. Thus, in order to achieve weight reduction of automobiles, there is an increasing demand for a steel sheet that has TS≧440 MPa, preferably TS≧500 MPa, more preferably TS≧590 MPa, and has excellent deep drawability and stretch flangeability. More specifically, there is a demand for a high-strength steel sheet that has a high Lankford value (hereinafter referred to as an r-value), such as average r-value≧1.2, preferably average r-value≧1.3, and a hole expansion ratio (hereinafter referred to as λ) of 80% or more. The Lankford value is a performance index of deep drawability, and the hole expansion ratio is a performance index of stretch flangeability.
As a technique for increasing strength, maintaining a high r-value, for example, Patent Literature 1 discloses a method for adding Ti or Nb for fixing solute carbon or solute nitrogen in an ultra-low carbon steel and adding a solid-solution hardening element, such as Si, Mn, or P, to the resulting interstitial atom free (IF) steel.
However, in accordance with such a technique for adding a solid-solution hardening element to an ultra-low carbon steel, the manufacture of a high-strength steel sheet having a tensile strength of 440 MPa or more requires a large amount of alloying element. For example, the addition of a large amount of Si results in the concentration of Si on a surface forms surface oxide during continuous annealing, this surface oxide deteriorating wettability. Si reacts with a minute amount of water vapor in the atmosphere to form a Si oxide on the surface. This results in poor coating wettability, uneven coating, and very low coating quality. The addition of a large amount of P deteriorates the anti-secondary working embrittlement by segregating of P in a grain boundary. The addition of a large amount of Mn results in a low r-value. Thus, there is a problem that the r-value decreases with strengthening of steels.
A method for strengthening a steel sheet other than the solid-solution hardening method described above may be a transformation strengthening. A dual phase steel sheet composed of mild ferrite and hard martensite generally has satisfactory ductility, excellent strength-ductility balance, and low yield strength. The dual phase steel sheet therefore has good press formability. However, the dual phase steel sheet has a low r-value and poor deep drawability. It is believed that solute C essential for the formation of martensite retards the formation of a {111} recrystallization texture, which is effective in increasing the r-value.
As a technique for improving the r-value of a dual phase steel sheet, for example, Patent Literature 2 discloses a method for performing box annealing at a temperature in the range of recrystallization temperature to Ac3 transformation point after cold rolling, heating the sheet to a temperature in the range of 700° C. to 800° C. to form a dual phase, and then quenching and tempering the sheet. Patent Literature 3 discloses a high-strength steel sheet that contains a predetermined amount of C, contains 3% by volume or more of at least one of bainite, martensite, and austenite in total, and has an average r-value of 1.3 or more.
However, the techniques described in Patent Literatures 2 and 3 require annealing for forming a cluster or precipitate of Al and N to grow a texture and thereby increase the r-value and heat treatment for obtaining the DP microstructure. Furthermore, the annealing process is based on box annealing, which requires a retention time as long as one hour or more. Box annealing takes a longer treating time than continuous annealing and increases the number of processes. This results in very low efficiency and productivity and poor economic viability in terms of manufacturing costs and causes many problems in the manufacturing process, such as frequent adhesion between steel sheets, temper coloring, and a decrease in life of a furnace inner cover.
Patent Literature 4 discloses a technique for improving the r-value of a dual phase steel sheet by optimizing the V content in connection with the C content. In accordance with this technique, before recrystallization annealing, C in the steel is precipitated as V carbide to minimize the amount of solute C and increase the r-value. Subsequently, the steel is heated in the ferrite(α)-austenite(γ) dual phase region to dissolve the V carbide and concentrate C in γ. Subsequently, martensite is formed in a cooling process to produce the dual phase steel sheet.
However, with respect to such a method of dissolving V carbide during annealing in the α-γ region, variations in dissolution rate may cause variations of the material property. Thus, the annealing temperature and the annealing time must be precisely controlled, and leaves a problem in the manufacture stability.
Patent Literature 5 discloses a technique for achieving both a high r-value and a dual phase by controlling the Nb content and the C content so as to satisfy 0.2≦(Nb/93)/(C/12)≦0.7 at a C content in the range of 0.010% to 0.050% by mass %. Patent Literature 5 also discloses a technique of combined addition such that the Nb content and the Ti content satisfy 0.2≦{(Nb/93)+(Ti/48)}/(C/12)≦0.7. In accordance with these techniques, solute C required for the formation of martensite remains in a hot-rolled steel sheet, and the r-value after annealing is increased by the effect of grain refinement of the hot-rolled steel sheet by the addition of Nb and the effect of decreasing the amount of solute C by the precipitation of NbC.
As a technique for improving the r-value and the α-value of a dual phase steel sheet, Patent Literature 6 discloses a technique for achieving both a high r-value and a high λ by controlling the Nb content and the C content so as to satisfy 0.2≦(Nb/93)/(C/12)≦0.7 at a C content in the range of 0.010% to 0.050% by mass, and controlling the ratio of the hardness of a second phase to the hardness of a ferrite phase in the range of 1.5 to 3.0.
The technique described in Patent Literatures 5 and 6 increase the r-value by the effect of grain refinement of the hot-rolled steel sheet by the addition of Nb and the effect of decreasing the amount of solute C by the precipitation of NbC. However, Nb is not only very expensive but also significantly retards the recrystallization of austenite and consequently increases the load in hot rolling. Furthermore, NbC precipitated in the hot-rolled steel sheet increases deformation resistance in cold rolling. For example, cold rolling at a rolling reduction of 65% as disclosed in the examples increases the load on a roll, increases the risk of trouble, decreases productivity, and results in a limited product size.